Comet - Like Planet
This term appears in a Soviet sci-fi novel and refers to a hypothetical type of small planet. It does not have a proper equivalent term in science literature and in Western sci-fi publications. As one will see below, such celestial bodies are possible to exist in young or altered solar systems. Most users of Universal Sandbox have experienced comet-like planets. Overview A comet-like planet is a planet-like celestial body, big enough to have a spherical shape but too small to retain a dense atmosphere at high temperatures. It contains high amounts of volatiles, like water ice and other gasses in solid form. When the planet gets close enough to a source of heat (a star or an artificial sun), temperature increases dramatically. The planet forms a large atmosphere, which is unable to hold in position. The planet then loses its volatiles, forming a giant coma, like a comet. As the process continues, the planet gradually loses its volatiles. The Plot The novel is called Star Of Legends. In the years 2600, humans settled on Sirius. The system is young and many planets had not had time to reach a state of equilibrium. There still are Hot Planets (which the author calls torch planets) as well as giant comets, some of them with cores the size of Pluto. When the novel was written, people thought that Pluto had a diameter of 5800 km. Also, some planets have satellites with too high concentrations of solidified gasses, which are outgassing. In the novel, Sirius has an outer gas giant, named Terak, which is roughly the mass of Brown Dwarfs. Scientists manage to initiate nuclear fusion inside Terak, turning it into an Artificial Sun. As this happens, many of Terak's moons, composed mainly of water ice and solidified gasses, start to become like comets, creating long tails that sometimes reach over 1 AU in length. At some point, scientists argue that also Jupiter and Saturn can be transformed into artificial suns, but that their moons could be destroyed as they start behaving like comets. Natural Occurrence Even if no telescope ever detected such a planet, there is a significant chance that they exist or can be created in future. *There is a possibility that comet-like planets form inside planetary systems just like planets, but lose their volatiles over time. Therefore, we might find them in very young solar systems. After losing their volatiles, they become rocky asteroids. *Comet-like planets can arrive from a Kuiper Belt or an Oort Cloud. By interacting with the gravity of one or two gas giants, the planet might end-up close enough to the star. *When a star gets old and increases its brightness (for example when it becomes a red giant), icy moons and Kuiper Belt objects can experience overheating. If large enough, they can become comet-like planets. *When an artificial sun is created, it can overheat icy moons and planets too. In the novel, planet Terak is located at 40 AU from Sirius. Average temperature was below -240 C or 30 K before the planet was transformed into an artificial sun. After the transformation was completed, its moons experienced temperatures between -170 and 2000 C. One possible former comet-like planet in the Solar System is Ceres. Scientists consider that Ceres was once an icy world with a subsurface ocean, which migrated to the Main Asteroid Belt. Exposed to higher temperatures, it lost its water. Surprisingly, Ceres has a radius similar to Pluto's solid core. If we move Pluto to the inner Solar System, it will become a comet-like planet. After exhausting its volatiles, its core, around the size of Ceres, will remain exposed. Structure According to the novel, the structure of a comet-like planet is different, depending on average temperature and length of thermal exposure. Comet-like planets orbiting Terak are very active, even eruptive, while comet-like planets and moons orbiting Sirius are far less active because they had been exposed to heat for longer time. Young comet-like planets To understand the behavior of a young comet-like planet, we should think at what would happen if we take Pluto from its current orbit and put it at 1 AU from the Sun (at Earth's orbit). Pluto has large deposits of volatiles on its crust (methane, carbon monoxide, nitrogen and ammonia). These gasses will evaporate very fast, creating a large atmosphere, larger then its diameter. As this will happen, it will also create a powerful greenhouse effect around the planet, increasing temperature even further and forcing all remaining gasses to evaporate. Exposed to the solar wind, Pluto will lose its atmosphere at a very fast rate. Escaping atmosphere will create a huge tail, stretching even as far as the orbit of Mars. The continuous evaporation process will keep surface temperature low for a while. On the other hand, the greenhouse effect will continue increasing temperature, stimulating evaporation. Such a long tail will interact with the solar wind, creating a stream of charged particle that will influence all passing spaceships. After some time, the surface gasses will be exhausted. Pluto's icy crust will then melt. It appears that Pluto's crust is actually made of a mixture of water ice, ammonia and possible other compounds. Again, given the low gravity, Pluto will be unable to sustain water in its atmosphere and continue to behave like a giant comet. However, since water evaporates at higher temperatures, the tail will not be as large as before. The process will last much longer then sublimation of solidified gasses. In the end, water will be exhausted. What will remain behind is Pluto's rocky core, tholins and salts. The resulting celestial body will be something similar to Ceres. In fact, scientists suggested that Ceres was once covered by an ocean which evaporated. Aged comet-like planets In time, impurities found in the ices (tholins, salts and rocks) tend to be deposited on the surface, forming a protective coat. This process reduces sublimation, but does not stop it completely. Limited outgassing still occurs, maintaining a tenuous atmosphere. After losing almost all volatiles, a comet-like planet has its surface covered with large amounts of boulders. This is similar to the surface of many near-Earth asteroids, which some scientists suspect to be extinct comets. Terraforming Comet-like planets are terraformable. The same principles that are suitable for Solar System's outer moons can be used here too. They even have the advantage of already having an atmosphere and not requiring greenhouse gasses. The best way to terraform such a planet is to use Ground Insulation. Ground insulation requires coating the planet with an insulation layer, which will protect the crust from heating. This type of terraforming is not permanent, as no insulation is perfect and the core might still be hot. However, melting process can be slowed enough so that the planet can be habitable for thousands of years. The crust of a comet-like planet is already heated to the boiling/sublimation point of its volatiles. Applying a ground insulation layer can be difficult while melting and outgassing processes occur. Also, the planet will not be able to sustain an atmosphere for long time. It can still hold a dense enough atmosphere for a few thousand years. Quasi - Equilibrium This is a different approach to comet-like planets. When the planet is heated, it first becomes an Oceanic Planet, with a lot of ice floating and with a dense and huge atmosphere and a massive tail. For many years, there is an equilibrium between the volatiles entering the atmosphere and those escaping. Water will evaporate gradually. In the same time, the ocean will release gasses like carbon dioxide, methane and ammonia. As water is exposed to solar radiation, it will break into hydrogen and oxygen. So, the atmosphere will be constantly supplied with gasses and water vapors. It will also be lost to space with a similar speed. The atmosphere will be very large, over twice the planet's diameter. Methane, ammonia and carbon dioxide are toxic in high concentrations. These gasses can be transformed into oxygen, Atomic Carbon (which will remain on the ocean's floor), nitrogen and hydrogen (which will fast escape into space) by plants or by huge artificial installations. Oxygen and nitrogen will also be lost in space and with a similar speed. If humans can transform gasses released by the ocean fast enough to replenish atmosphere losses, the planet can be habitable, even if it will require continuous maintenance. Category:Terraformed models